CORONA
“BIG META”
A Big “Metamaterial”
Broadband Acoustic Resonator Array Enclosure
For a Midrange Driver
Designed and Built by
Meredith A. Cargill
October 14, 2020
DESIGN GOALS
To implement in midrange the same kind of device as
KEF has introduced for the tweeter in its LS50 Meta
To finish before the end of the year a novel design that
commemorates 2020 as the year of the Novel Coronavirus
DESIGN IN BRIEF
Use many discrete plastic pipes to construct a
broadband acoustic resonator behind the driver,
with the pipes radiating magnificently out from the center.
The build is 70.5” high and 88.5” wide. The stand lifts the driver to 42” from the floor, making the whole structure 78.5” tall.
Before I give the details of how I built it, and the acoustic measurements, let me give the theory.
THE PHYSICS:
Broadband Acoustic Resonator Array
Broadband Acoustic Resonator Array
Researchers in acoustic engineering have been exploring, over the past fifteen or twenty years, devices they have come to call “metamaterials” and “metasurfaces.” These are structures that can be used to control the transmission or reflection of sound in place of traditional solutions that involve thick layers of material or large, heavy structures, such as fiberglass bats and quadratic diffusers. The engineering goal is to reduce the size of the devices and constructions needed for passive reduction of unwanted sound in rooms and other spaces.
They typically, if not universally, implement these solutions using multiple resonating tubes or volumes. Their obsession is proving that their structures are thinner or smaller than the traditional solutions while being as effective at reducing acoustic energy. For instance, in a 2017 paper in Physical Review X, “Ultrathin Acoustic Metasurface-Based Schroeder Diffuser,” Yifan Zhu, et al., describe connecting multiple small Helmholz radiators to form a sheet or panel that scatters sound in the same way as a Schroeder quadratic diffuser, but is only one-tenth as thick. https://journals.aps.org/prx/pdf/10....sRevX.7.021034 In a 2017 paper in Annual Review of Materials Research, “Sound Absorption Structures: From Porous Media to Acoustic Metamaterials,” Min Yang and Ping Sheng describe the “realization of broadband absorbers with discrete arrays of resonators” (107). http://sheng.people.ust.hk/wp-conten...Absorption.pdf Instead of Zhu’s pockets with openings that produce Helmholtz resonances, Yang and Sheng use a package of tubes that resonate according to their length. The out-of-phase reactive behavior of the tubes cancels the sound impinging on the device, so that the sound is “absorbed” by it, with the same effect as a much larger volume of porous material that absorbs sound by impeding it. Even though each tube individually produces a very narrow-band effect, they get a broadband effect (at least two octaves) by using an array of 16 tubes of different length (110).
The engineers at KEF have used the principles described by Yang and Sheng to build a device to absorb the energy coming from the back side of the tweeter diaphragm in their concentric Uni-Q driver, which has the dome tweeter mounted in the center of a mid-woofer. Sebastien Degraeve and Jack Oclee-Brown describe the device in their June, 2020 paper for the Audio Engineering Society, “Metamaterial Absorber for Loudspeaker Enclosures.” It is a disk 11mm thick, the diameter of the magnet structure of the driver (about 5"). It thus resembles in physical shape a sheet of sound-absorbing mat or felt that they might have used to damp that energy by mounting it to the inside of a conventional sealed box enclosure. But it is instead an injection-molded array of tiny tubes cleverly folded to fit into the neat disk shape. The disk mounts to the back of the magnet structure of the driver. The rearward output from the tweeter is directed by a channel through the midrange motor to the center of the disk, where it meets the mouths of all the tubes. The device absorbs the energy by cancellations due to reactive resonances, and is more effective at doing so than any known resistive absorbing material of comparable thickness would be. Thus, it fits the description of what the researchers call a “metamaterial.”
Because the word “metamaterial” is ultra-technical and mystifying and unfamiliar to any audience outside acoustics research, I don’t like it. You can find my rant in the TechTalk thread about KEF’s device, here: http://techtalk.parts-express.com/fo...chnology/page3 Once it crosses over from acoustics research to KEF’s marketing, it sounds less like science than like hype. We could look to a quotation like the following for help understanding the idea: “Merely by relating himself to what is, man places and faces beings in their Being. Seen in the light of what is, the facing, the idea of beings always goes beyond beings. … But the presence of what is present is not finally and also something we face, rather it comes before. Prior to all else it stands before us, only we do not see it because we stand within it. It is what really comes before us. The facing, the idea of what is, judged from what is, is always beyond what is--µetá.” That is Martin Heidegger trying to turn phenomenology into metaphysics (What Is Called Thinking? Harper, 1968, 97-98). Does it help?
A more appropriate, less confusing, more informative term for KEF’s device is a broadband acoustic resonator array. To quote Degraeve and Oclee-Brown, it is “a structure containing many high-Q resonators optimized to provide a wide bandwidth of overall absorption” (2). The authors go overboard in the direction of mystification when they say “acoustic metamaterials can deliver unconventional effective properties without the constraints normally imposed by nature” (2). Without the constraints imposed by nature? They make it sound as if a metamaterial is miraculous or supernatural. (Let the marketing metaphysics begin!) The principle it works on is entirely natural. The principle also is not anything new, because resonators are familiar in speakerbuilding. We use Helmholtz resonators when we build boxes with ports in them, and we use quarter-wave resonators when we build “transmission line” cabinets. Neither is there any new material involved. Not only is injection-molded plastic not new, the same device could also be made out of cast metal or carved wood. It is misleading to suggest, by calling it some kind of “material,” that the idea is about the material. What is new about it is implementing an array of a large number of resonators to achieve a broadband effect.
In a thread at DIYAudio.com, Sin Phi claims to have produced an equivalent to KEF’s disk, suitable for a tweeter, using 3D printing. https://www.diyaudio.com/forums/mult...terials-7.html
What I have done in this design, Corona, is simply scale up KEF’s device so that it functions at longer wavelengths, appropriate for a midrange driver. The same concept could be used for a woofer, if one is willing to accommodate the necessary pipe lengths.
One thing I hope to demonstrate is that the concept is not about a material that can be used to line or stuff a conventional box, or even build a box out of; it is instead about an enclosure design that damps internal energy by its structure. It is, thus, not an alternative to fiberglass or Dacron fiber or denim felt or long-fiber wool, which we use to absorb and impede sound inside an enclosure; it is instead a whole different kind of enclosure, which makes it an alternative to a sealed box, ported box, open baffle, band pass, etc. As such, it strikes me as something novel.
THE CORONA
I have kept KEF’s idea of laying the tubes out in a plane, radiating outward, with their mouths forming a circle in the center. Rather than try to fold the tubes to be compact, I have chosen to let them fly outward, something like the points of a crown. Since the Latin word for a crown, ‘corona,’ has been used to describe other things with this shape, I have used it as the name for this design. One can see, for instance, the Sun’s corona during a total eclipse.
And one can see coronas painted behind the heads of the divine beings in Christian icons and religious paintings.
More familiar to everyone this year is the corona virus, especially the new one that appeared in 2019. A corona virus gets its name from how it looks under a microscope—like a circle with spikes sticking out all around it. In 2-D silhouette, it looks like a crown.
Once I had the idea of a corona-shaped loudspeaker, I could not resist building it while in the year of the Novel Coronavirus (Covid-19) pandemic.
Since the pandemic has prevented us from holding shows for our speaker designs, such as the contest at Parts Express Midwest Audiofest, I presume this will be an online-only presentation. I am thankful that this means I can get by with building only one, as proof of concept, rather than having to build a stereo pair, and be judged by its stereo imaging. I also did not have to worry about whether it would fit in my van, or be damaged in transport, or how difficult it would be to set up in the show space.
Build details, and acoustic measurements, will come in additional posts.
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